Structure/activity relations of agonists and antagonists for the N-methyl-D-aspartate receptor have been investigated by measuring the ability of a large range of substances to inhibit binding of [3H]2-amino-5-phosphonopentanoate to rat brain membranes. A major difference between optimum structures for agonist and antagonist activity lay in the differential effectiveness of sulphonic and phosphonic acid groups as the omega-acidic terminal in these two types of compound. The sulphonic acid moiety was an effective omega-acidic terminal in short chain agonists, but not in longer chain antagonists, while the phosphonic acid group was the most effective omega-acidic terminal in longer chain antagonists, but was only very weakly active in short chain agonists. It is proposed that the binding site of the omega-acidic terminal of antagonists is different from that for the omega-acidic group of agonists. Other structural features conducive to effective interaction of ligands with the receptor are discussed.